I have experienced this anomaly before with some of my clients and found we needed to uninstalled all add-ins (unsure if others were installed as none were showing but it did find and uninstall excel add-ins). Then installed the latest add-in using the ''run as administrator'' option and selected the option to install for all users so that the main would be within root locations of windows and thus be accessible to all users and the application. The add-in was then detected to be installed on the system and working normally within Excel. I have also seen it where the Excel needed to be repaired. The way that would be accomplished is to run the installer again and when the installer starts it will give the option to repair or uninstall. Running the repair has corrected issues and allowed the add-in to work too. I would try these options first and then if neither work, open a case with your support group for ALM.
Hope this helps,